1. Field of the Invention
The present invention relates to a focus control amount determination apparatus and method for determining a focus control amount for an imaging optical system in an automatic focus mechanism of an imaging apparatus. It also relates to an imaging apparatus that includes the focus control amount determination apparatus.
2. Description of the Related Art
In imaging devices, such as digital still cameras, digital video cameras, and the like, autofocus (AF) mechanisms that causes the focus lens to come into focus on a predetermined subject have been widely use. The AF mechanisms include the active system in which the distance from the imaging apparatus to the subject is measured by irradiating infrared light from the imaging device to the subject, and detecting the angle of the infrared light reflected back to the imaging device, and the position of the focus lens is set where the focus lens comes into focus on the object at the measured distance, and the passive system in which the focused status is detected by processing image signals outputted from the imaging means of an imaging apparatus, and the focus lens is set at a position where best focus status is obtained.
The passive AF mechanisms which are widely known in the art include: the phase detection system in which the focusing status is determined from the amount of lateral displacement, and the contrast detection system in which the focusing status is determined from the contrast of the image. In the contrast detection AF mechanism, the focus lens is moved in a stepwise manner within the working range of focusing (e.g., from near end to infinity), and image data are obtained from the imaging means every time the focus lens is moved stepwise, thereby the focus lens is set at a position corresponding to a maximum focus evaluation value (contrast value) of the obtained image data.
When taking a portrait, it is desirable that the focus is adjusted on the face of the person, the major subject of the image. In a conventional AF control system, in which the AF area, the subject area for distance measurement and focus determination, is fixed in the center of the imaging screen. Therefore, if two persons, the major subjects, present on the screen, each on each side of the center, for example, the focus is adjusted on the background included in the AF area located in the center of the screen, which causes a what is known as “clearing in midsection” and the image obtained is out of focus for the two persons.
Consequently, the following methods for preventing such clearing in midsection phenomena are known in the passive system AF mechanism. In one of the methods, as described, for example, in Japanese Unexamined Patent Publication No. 11 (1999)-146405, a flesh color area indicating a person is detected from the image signals outputted from the imaging means, and the focus lens is set at a position where best focus state is obtained using the detected flesh color area as the AF area. In another method, as described, for example, in Japanese Unexamined Patent Publication No. 2004-317699, a characteristic region is extracted from an image on the imaging screen, then a face is discriminated from the extracted characteristic region, and if the size of the discriminated face is greater than or equal to a predetermined value, contrast evaluation values employed in the contrast detection system are calculated, using the area including the face as the AF area.
In the method described in Japanese Unexamined Patent Publication No. 11 (1999)-146405, however, there may be a case that the focus is adjusted on a flesh color region of an image other than the face, since the flesh color region of the image is fixedly determined as a human area. In the method described in Japanese Unexamined Patent Publication No. 2004-317699, there may be a case that the focus is not adjusted on the face of a person, since the AF area also includes the area other than the face although the face is discriminated, and if the contrast in the region is great, the focus evaluation value becomes maximum in the region and the focus is adjusted thereon.